The present invention relates generally to the art of hydrostatic transmissions and more particularly relates to improvements in the control of a pump for a hydrostatic transmission.
In the typical hydrostatic transmission utilized for a vehicle drive, the driving speed of the transmission is varied by controlling the displacement of the pump of the transmission. In this type of transmission the pressure delivered by the pump is not directly controlled, but is a function of the load on the vehicle. The displacement of the pump is normally controlled through some mechanical linkage which, to a certain extent, will simulate a vehicle throttle. That is, the mechanical linkage can either be hand or foot operated to increase or decrease the transfer speed of the transmission. With such a transmission it is often desirable, particularly when displacement is controlled through a hand lever, to have an additional control which will override the main control and which is operated through a pedal so that in an emergency situation the operator's natural tendency to depress a clutch or brake pedal will operate the additional control and bring the hydrostatic transmission into a neutral condition. Such a control also provides a convenient method of controlling the transmission when small movements are required such as when attempting to hook an agricultural tractor to an implement. This control of the transmission to provide small increments of movement of the vehicle is generally referred to as an inching control. In a typical hydrostatic transmission such a secondary control merely dumps fluid pressure from the pressure line between the transmission pump and motor to a charge circuit or reservoir or to the return line between the pump or motor so that the fluid pressure in the pressure line between the pump or motor is reduced to a value substantially equal to the value of the pressure in the return line between the pump and motor.
It is also desirable in the hydrostatic transmission to limit the maximum pressure of the fluid delivered to the pump. This is generally accomplished by providing a pressure relief valve in the pressure line between the pump and motor which is responsive to a predetermined maximum pressure to drain excess fluid to a reservoir or charge circuit or to the return line between the motor and pump. While such an arrangement does limit maximum pressure, it does not in any way cure the condition creating the maximum pressure and hence results in power loss due to heat generation.